# std::set

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< cpp‎ | container

Defined in header `<set>` |  |
template< class Key, class Compare = std::less<Key>, class Allocator = std::allocator<Key> > class set; | (1) |
namespace pmr { template< class Key, class Compare = std::less<Key> > using set = std::set<Key, Compare, std::pmr::polymorphic_allocator<Key>>; } | (2) | (since C++17)

`std::set` is an associative container that contains a sorted set of unique objects of type `Key`. Sorting is done using the key comparison function Compare. Search, removal, and insertion operations have logarithmic complexity. Sets are usually implemented as Red–black trees.

Everywhere the standard library uses the Compare requirements, uniqueness is determined by using the equivalence relation. In imprecise terms, two objects a and b are considered equivalent if neither compares less than the other: !comp(a, b) && !comp(b, a).

`std::set` meets the requirements of Container, AllocatorAwareContainer, AssociativeContainer and ReversibleContainer.

### Template parameters

| This section is incomplete Reason: Add descriptions of the template parameters.

### Member types

Type | Definition
`key_type` | `Key`
`value_type` | `Key`
`size_type` | Unsigned integer type (usually std::size_t)
`difference_type` | Signed integer type (usually std::ptrdiff_t)
`key_compare` | `Compare`
`value_compare` | `Compare`
`allocator_type` | `Allocator`
`reference` | value_type&
`const_reference` | const value_type&
`pointer` |
`Allocator::pointer` | (until C++11)
std::allocator_traits<Allocator>::pointer | (since C++11)
`const_pointer` |
`Allocator::const_pointer` | (until C++11)
std::allocator_traits<Allocator>::const_pointer | (since C++11)
`iterator` | Constant LegacyBidirectionalIterator to `value_type`
`const_iterator` | LegacyBidirectionalIterator to const value_type
`reverse_iterator` | std::reverse_iterator<iterator>
`const_reverse_iterator` | std::reverse_iterator<const_iterator>
`node_type` (since C++17) | a specialization of node handle representing a container node
`insert_return_type` (since C++17) | type describing the result of inserting a `node_type`, a specialization of template<class Iter, class NodeType> struct /*unspecified*/ { Iter position; bool inserted; NodeType node; }; instantiated with template arguments `iterator` and `node_type`.

### Member functions

(constructor) | constructs the `set` (public member function)
(destructor) | destructs the `set` (public member function)
operator= | assigns values to the container (public member function)
get_allocator | returns the associated allocator (public member function)
##### Iterators
begincbegin (C++11) | returns an iterator to the beginning (public member function)
endcend (C++11) | returns an iterator to the end (public member function)
rbegincrbegin (C++11) | returns a reverse iterator to the beginning (public member function)
rendcrend (C++11) | returns a reverse iterator to the end (public member function)
##### Capacity
empty | checks whether the container is empty (public member function)
size | returns the number of elements (public member function)
max_size | returns the maximum possible number of elements (public member function)
##### Modifiers
clear | clears the contents (public member function)
insert | inserts elements or nodes(since C++17) (public member function)
insert_range (C++23) | inserts a range of elements (public member function)
emplace (C++11) | constructs element in-place (public member function)
emplace_hint (C++11) | constructs elements in-place using a hint (public member function)
erase | erases elements (public member function)
swap | swaps the contents (public member function)
extract (C++17) | extracts nodes from the container (public member function)
merge (C++17) | splices nodes from another container (public member function)
##### Lookup
count | returns the number of elements matching specific key (public member function)
find | finds element with specific key (public member function)
contains (C++20) | checks if the container contains element with specific key (public member function)
equal_range | returns range of elements matching a specific key (public member function)
lower_bound | returns an iterator to the first element not less than the given key (public member function)
upper_bound | returns an iterator to the first element greater than the given key (public member function)
##### Observers
key_comp | returns the function that compares keys (public member function)
value_comp | returns the function that compares keys in objects of type `value_type` (public member function)

### Non-member functions

operator==operator!=operator<operator<=operator>operator>=operator<=> (removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(C++20) | lexicographically compares the values of two `set`s (function template)
std::swap(std::set) | specializes the std::swap algorithm (function template)
erase_if(std::set) (C++20) | erases all elements satisfying specific criteria (function template)

### Deduction guides | (since C++17)

### Notes

The member types `iterator` and `const_iterator` may be aliases to the same type. This means defining a pair of function overloads using the two types as parameter types may violate the One Definition Rule. Since `iterator` is convertible to `const_iterator`, a single function with a `const_iterator` as parameter type will work instead.

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_containers_ranges` | `202202L` | (C++23) | Ranges construction and insertion for containers

### Example

Run this code

    #include <algorithm>
    #include <iomanip>
    #include <iostream>
    #include <iterator>
    #include <set>
    #include <string_view>

    template<typename T>
    std::ostream& operator<<(std::ostream& out, const std::set<T>& set)
    {
        if (set.empty())
            return out << "{}";
        out << "{ " << *set.begin();
        std::for_each(std::next(set.begin()), set.end(), [&out](const T& element)
        {
            out << ", " << element;
        });
        return out << " }";
    }

    int main()
    {
        std::set<int> set{1, 5, 3};
        std::cout << set << '\n';

        set.insert(2);
        std::cout << set << '\n';

        set.erase(1);
        std::cout << set << "\n\n";

        std::set<int> keys{3, 4};
        for (int key : keys)
        {
            if (set.contains(key))
                std::cout << set << " does contain " << key << '\n';
            else
                std::cout << set << " doesn't contain " << key << '\n';
        }
        std::cout << '\n';

        std::string_view word = "element";
        std::set<char> characters(word.begin(), word.end());
        std::cout << "There are " << characters.size() << " unique characters in "
                  << std::quoted(word) << ":\n" << characters << '\n';
    }

Output:

    { 1, 3, 5 }
    { 1, 2, 3, 5 }
    { 2, 3, 5 }

    { 2, 3, 5 } does contain 3
    { 2, 3, 5 } doesn't contain 4

    There are 5 unique characters in "element":
    { e, l, m, n, t }

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 103 | C++98 | iterator allows modification of keys | iterator made constant
LWG 230 | C++98 | `Key` was not required to be CopyConstructible (a key of type `Key` might not be able to be constructed) | `Key` is also required to be CopyConstructible

### See also

multiset | collection of keys, sorted by keys (class template)
unordered_set (C++11) | collection of unique keys, hashed by keys (class template)
flat_set (C++23) | adapts a container to provide a collection of unique keys, sorted by keys (class template)

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